The direct oxidation of ethylene to ethylene oxide by molecular oxygen is well-known and is, in fact, the method used currently for commercial production of ethylene oxide. The typical catalyst for such purpose contains metallic or ionic silver, optionally modified with various promoters and activators. Most such catalysts contain a porous, inert support or carrier such as alpha alumina upon which the silver and promoters are deposited. A review of the direct oxidation of ethylene in the presence of supported silver catalysts is provided by Sachtler et al. in Catalyst Reviews: Science and Engineering, 23 (1&2), 127-149 (1981).
It is also well-known, however, that the catalysts and reaction conditions which are best suited for ethylene oxide production do not give comparable results in the direct oxidation of higher olefins such as propylene. The discovery of processes capable of providing propylene oxide by vapor phase direct oxidation in higher yields than are presently attainable thus would be most desirable.
Workers in the field have recognized for a number of years that the efficiency of a direct propylene epoxidation process catalyzed by a supported silver catalyst may be improved by introducing relatively small amounts of both a nitrogen oxide species such as NO and a volatile organic chloride such as ethyl chloride to the feedstream containing propylene and oxygen. See for example, U.S. Pat. No. 5,387,751 (Hayden et al.) and Canadian Pat. Nos. 1,282,772 (Thorsteinson) and 1,286,687 (Habenschuss et al.).
However, the addition of volatile organic chlorides into the feedstream has certain practical disadvantages. The use of an organic chloride, even at the ppm levels typically employed, adds significantly to the raw material costs associated with producing propylene oxide. Measures must be implemented to recover or trap any organic chloride in the effluent exiting the epoxidation reactor. Such recovery methods may involve the generation of ionic chloride species, which tend to accelerate corrosion of the construction materials used in the recovery section. Additionally, recovery of organic chloride is not always quantitative; unreacted or unhydrolyzed organic chlorides thus may escape from the process, contributing to air pollution.
Thus, it is readily apparent that the development of direct propylene epoxidation processes which do not require the addition of organic chlorides to the feedstream but which give satisfactorily high selectivity to propylene oxide would fulfill a great need in the field.